I'm building a home and want to have a passive solar greenhouse added to the south side. What configuration tips can you give?

Answer

The options you have here will depend a lot on your specific site, especially its orientation and shading. I will try to give you some general tips that apply to most sites, but please feel free to contact me with more information so I can make better recommendations – I am local to you in Louisville, CO.

You will first need to decide if you want to:

add passive solar as a “feature” like the greenhouse you mentioned above or if you will

incorporate passive solar design in your overall house.

If you are building the entire house from scratch, I would strongly recommend the second approach. Let’s look at both.

Adding a passive solar greenhouse

With this approach, you build a glass-enclosed extra room on the south side of your house. In summer you will open lots of its windows to stop it from overheating, but it will be a nice space for the dinner table when it cools down at night.

Its energy-saving features come to life only in winter: on sunny winter days it will heat up and you either mechanically pump the warm air into your main house or you can design your house in a way that the warm air flows into the main house and rises into the second floor by itself. On overcast winter days, the greenhouse will be very cool since glass insulates a lot less than regular walls, so you want to seal the greenhouse off from your main living space.

The advantage of this concept is that you can add the greenhouse to an existing residence, or if you are doing new construction anyway the main house does not have to follow passive solar design principles.

Building a true passive house

Here the idea is rather than building a greenhouse that collects excess heat on some winter days to design the entire residence in a way that no or minimal mechanical heating and cooling are needed. This is achieved fairly easily by orienting the house properly, and calculating the perfect balance between the size of windows, the length of overhangs and the amount of thermal mass inside the house.

As a result, your house will self-regulate the internal temperature. On hot summer days, the overhangs will keep the sun out of your house while the thermal mass in your floors and walls that cooled down during the night will keep the house cool over the day. In winter, the low standing sun reaches under the overhangs and shines deep into your house, warming it up and storing energy in the thermal mass again that can be dissipated at night or on overcast days.

Configurations that maximize passive solar energy

Now that we have looked at the two approaches, let’s look into configurations.

For the greenhouse approach, you clearly need to be on the south side with as little shading as possible. You need to use clear glazing with a high SHGC, which describes the percentage of the sun ray's energy that enters the room. Furthermore, you want to configure the greenhouse in a way that maximizes the exposure to the sun in winter where the sun is at a low angle, and minimizes it in summer so you have a chance of using your greenhouse in the warm season too.

To achieve this the best would be to have your greenhouse about four to eight feet wide along the entire south facade, the full height of the house (if your house is two stories, even better) with the roof of your house forming the roof of your greenhouse as well. The rooms of the house could open to the greenhouse with french doors whenever the temperature in the greenhouse is more comfortable. This way the greenhouse will also double as a sort of enclosed deck to many rooms. Alternatively, you can add the greenhouse more like a cube somewhere along the south facade, maybe connecting with a big sliding door to your living room. Ideally the room it connects to would be two stories high (if you have two stories) so that the warm air can rise by itself to the upper floor. This approach will work well but not result in as much energy savings as the first suggestion.

For the true passive house you will need to minimize windows to the north and reduce to east and west, while having large windows to the south. A rectangular building volume stretching east-west will work best, but in the Colorado climate many configurations are possible. Overhangs and room depths are balanced so that the summer sun stays out but the winter sun reaches all the way into the back of the room. Concrete floors work best to absorb, store and release the energy, and work great with radiant heat (which you may not need even more though if the house is properly designed). You may want to work with an architect or residential designer specializing in passive design to get all the components balanced just right.

As I mentioned in the beginning, not knowing your site and your needs for the house I was only able to give some general tips and recommendations. A site visit would allow me to give you much more applicable ideas, so feel free to contact my office. There is no charge for such a consultation.

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